Magnetic resonance imaging (“MRI”) is a well-known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities that are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.
During an MRI procedure, the patient is inserted into an imaging volume of a primary field magnet. The magnet generates a static magnetic field through that causes the vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons to tend to align with the direction of the static magnetic field. A transmitting antenna proximate to the imaging volume emits a pulse or pulses of radio frequency energy. The radio frequency energy has a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, that shifts the vectors of the nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency receiving antenna tuned to that frequency. The receiving antenna is typically positioned within the imaging volume proximate the patient. Linear, time-varying gradient magnetic fields are superimposed onto the static magnetic field to spatially encode the MR signals emitted by the nuclei and to define a particular image slice. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay.
The same antenna may act as the transmitting and receiving antenna. The MR signals detected by the receiving antenna are amplified, digitized and processed by the MRI system. Hydrogen, nitrogen, phosphorous, carbon and sodium are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By correlating the gradient magnetic fields and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient by known mathematical techniques, the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient's body may be determined. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest. Certain abnormalities in tissue, such as tumors, may be identified.
MRI can be of great assistance during medical procedures. For example, MRI has been used for pre-operative and postoperative imaging to identify and assess the condition of tissue of interest. MRI has also been used during fine-needle aspiration cytology to help the doctor guide the needle to the site of interest, such as a tumor. See, for example, U.S. Pat. No. 6,208,145 B1, assigned to the assignee of the present invention and incorporated by reference, herein. MRI has also been used in stereotactic neurosurgery. The advance of other instruments, such as a catheter or an endoscope, can also be followed and guided to a site of interest by MRI. See, for example, U.S. Pat. No. 6,249,695 B1 and U.S. Pat. No. 5,647,361, which are both assigned to the assignee of the present invention and incorporated by reference, herein. A catheter guided to a site of interest by MRI can be used in the treatment of tissue, such as a tumor, by delivering medication, isotopes or other such treatments, for example. MRI may also be used to monitor the affect of a treatment on the tissue, as the treatment is being conducted. See, for example, U.S. Pat. No. 6,208,145 B1 and U.S. Pat. No. 6,280,383 B1, both assigned to the assignee of the present invention and incorporated by reference herein.
To conduct surgery, the imaging volume needs to be large enough for one or more surgeons and other medical personnel to have clear and unimpeded access to the patient. In U.S. Pat. No. 6,208,145 B1, assigned to the assignee of the present invention and incorporated by reference herein, open MRI assemblies are disclosed wherein a physician or other medical personnel may conduct activities within the frame of the assembly, adjacent to the patient. In one embodiment, a ferromagnetic frame comprises two opposing vertical ferromagnetic plates connected to two opposing ferromagnetic pole supports. Opposing ferromagnetic poles extend towards each other, from the pole supports. Resistive or superconductive coils wrapped around the poles provide magnetic flux through the ferromagnetic frame. An imaging volume is defined between the opposing poles, for receiving at least a portion of a subject for imaging. The poles are above and below the patient. The regions around the sides of the patient are open, decreasing any claustrophobic reaction the patient may experience. In addition, medical personnel may access the patient through the open side regions, enabling performance of medical procedures on the patient while the patient is within the imaging volume and undergoing MRI.
The magnet assembly of the MRI system may define a room for conducting a medical procedure and may be large enough to contain an entire surgical team. The Quad™ 7000 and Quad™ 12000 Open MRI Systems, available from FONAR Corporation, Melville, N.Y., are also suitable for performing surgery and other medical procedures.
MRI systems in accordance with U.S. Pat. No. 6,208,145 B1 provide about 18–19 inches of open space between the opposing poles of the assembly. Additional room for the doctor to maneuver proximate imaging volume during a medical procedure may be provided by tapering the upper pole, as described in U.S. Pat. No. 6,346,816 B1, assigned to the assignee of the present invention and incorporated by reference herein. Additional room may also be provided by tapering portions of the bottom of a canopy of insulative material which typically covers the upper (and lower) pole and accessories, as described in U.S. Ser. No. 09/919,286, filed on Jul. 31, 2001 also assigned to the assignee of the present invention. The recessed or tapered portions enable the doctor or other such personnel in the room to lean into the imaging volume during a medical procedure. Two recessed portions are typically provided, symmetrically arranged around the periphery of the canopy.
Despite these improvements in the design of open MRI magnet assemblies to make them more conducive for conducting surgery, it would be advantageous to provide further room for medical personnel to access a patient in an imaging volume of an MRI magnet assembly.
In U.S. Pat. No. 6,029,081, several magnet assemblies are disclosed wherein one or both poles may be moved with respect to the imaging volume to provide room to conduct medical procedures. In one embodiment, the entire magnet assembly is supported on rollers or wheels on a track and can be moved along the track, away from the patient, when necessary. The assembly may be moved back into an imaging position when MR images are needed. In another embodiment, a portion of a magnet supporting an upper pole is rotatable or pivotable to move the upper pole out of the imaging volume. In another embodiment, a portion of the magnet and the pole are raised by a lifting mechanism to enlarge the imaging volume. The lifting mechanism is movable along a track on the ceiling to move the magnet and pole out of the way. In another embodiment, the magnet and both of the supported poles are separable in three directions, horizontally. These designs are complex and impractical. Movement of the ferromagnetic elements would require that the magnetic field be shut down in all but the smallest magnets. After such a shut down, when MRI is desired, the magnet would need to be reassembled and allowed to warm up, causing long delays during the medical procedure.